The hypothesis being tested in this study is that implantation of a human allograft during pediatric cardiac surgery causes an antibody response in the recipient, which accelerates allograft calcification and dysfunction. The specific aim of this protocol is to determine if allograft implantation induces an antibody response in the recipient manifested as an elevated panel-reactive antibody (PRA) in the recipient. To date, the investigators have prospectively analyzed the immune response of nine children, ages two months to ten years, who required a non-valved allograft patch to alleviate stenosis caused by a congenital cardiac defect. The diagnoses in these patients were: aortic arch obstruction in three; transposition in two; complex single ventricle in two; and one patient each with pulmonary atresia, intact ventricular septum, and tetralogy of Fallot. Levels of HLA panel-reactive antibody (PRA) in the serum of each patient were measured prior to surgery, one week after, one month after, and three months after allograft placement. PRA measurements were negligible preoperatively and at one week after surgery. However, within one month, all patients showed a vigorous immune response. From this preliminary data, the investigators have concluded that, similar to valved allografts, cryopreserved non-valved allografts induce a strong HLA antibody response that broadens in reactivity within three months of transplant. This response represents antibody to both HLA class I and class II antigens. These findings suggest that even small pieces of cryopreserved non-valved allograft tissue, including monocusp pulmonary allografts, exhibit sufficient immunogenicity to induce a strong antibody response that could be damaging to the tissue.